US3715672A

US3715672A - Sonic detection apparatus

Info

Publication number: US3715672A
Application number: US00199868A
Authority: US
Inventors: H Suter
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1970-04-09
Filing date: 1971-11-18
Publication date: 1973-02-06
Anticipated expiration: 1990-02-06

Abstract

An improved swimmer and floater detector having a sonic generator and receiver and a shift detector indicating the doppler frequency difference between the generated and received sonic waves within a guarded volume of flowing water. The shift detector output is passed to a plurality of adjacent narrow bandpass filters, each being connected via a respective gaincontrolled amplifier to a common multiple frequency meter. The amplifier gain control is non-proportional to the output amplitude and is operative in two modes to steadily increase or decrease the gain when the output is below or above threshold respectively.

Description

eel- 9 $55 $8 24514 7 xar ants-1,672.: 7 X J 3 11:" Unite l 1111 3,715,672 m 1 51 Feb. 6, 1973 [541 SONIC DETECTION APPARATUS 3,585,401 6/1971 Karl ..307/290 3,534,274 10/1970 Robinson ..328/l64 [75] Inventor: Henry Ham"), 3,538,444 11/1970 Adlhoch ..307/235 [73] Assignee: The United States of America as Primary Examiner james Lawrence represented by the Secretary of the Assistant Examiner-Harold A. Dixon y Attorney-R. S. Sciascia et a1. [22] Filed: Nov 18, 1971 ABSTRACT [21] Appl' 199368 An improved swimmer and floater detector having a A D ta sonic generator and receiver and a shift detector in- Related U S pp a dicating the dopple r fr eguency difference between the [62] Division of 31,042 Apr 9, 1970 generated and reeived sonic waves within a guarded Pat No 3 646 506 volume of flowmg water. The shift detector output is 40 258 passed to a plurality of adjacent narrow bandpass fil- [52] US. Cl ..328/150, 3 ters, each being connected via a respective gaimcom [5 l] IIBt. Cl. ..'.H03k 17/00. trolled amplifier to a common multiple frequency Fleld Search 266; 307/290 'ter. The amplifier gain control is non-proportional to 328/150 the output amplitude and is operative in two modes to steadily increase or decrease the gain when the output References Cited is-below or above threshold respectively.

I UNITED STATES PATENTS 7 e1a1ms,3"1i;aw1i;gngures 2,749,537 6/1956 Loudon etal. ..340/258 A 2,903,683 9/1959 Bagno, ..340/258 A 3,294,981 12/1966 Bose ..307/290 F ROM 24 37125672, on maze/1 50 SONIC DETECTION APPARATUS CROSSREFERENCES TO RELATED APPLICATIONS This is a division of application Ser. No. 31,042, filed Apr. 9, 1970 now US. Pat. No. 3,646,506.

STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

CROSS-REFERENCES TO RELATED APPLICATIONS This is a division of application Ser. No. 31,042, filed April 9, 1970 now US. Patent No. 3,646,596.

BACKGROUND OF THE INVENTION The invention relates generally to improvements in underwater detection systems, and more particularly to an improved, continuous wave, doppler sonar system having a gain compensation circuit for each bandpass filter. 1

In the past doppler sonar systems have been used in rivers for detecting the approach of swimming or floating objects to guard against sabotage of river installations. One type of equipment uses adjacent narrow bandpass filters to monitor the amplitudes of doppler shifts within certain frequency bands. Each band has an ambient level characteristic of the condition of the guarded volume with respect to stream velocity. The presence of a swimmer or floater causes an increase in amplitude over the ambient level in one or more bands. The frequency shift for swimmers is characteristically more pronounced than that associated with floating targets. If an alarm is to be triggered by an excess of activity within any band, it is important that the registered level be compared with ambient or non-target conditions. With the prior equipment a steady change in stream velocity due to storm or tide necessitated frequent manual recalibration by trained personnel of the ambient levels in each band. Without such re-adjustment, increases stream velocity could trigger a false alarm. Moreover, each filtered band required its own amplitude meter. The size, weight and simultaneous readability of numerous meters seriously limited the number of discrete bandwidths which could be effectively monitored.

SUMMARY or THE INVENTION Accordingly, one of the objects of the invention is to improve the operating characteristics of the prior doppler swimmer detector by providing automatic adjustment of the bandpass filtersto compensate for variations in the doppler frequency spectra caused by changes in the river velocity. Another object of the invention is to prevent accidental triggering of the alarm by gradual changes in the ambient level. A further object is to make the rate and direction of change in the gain of each amplified bandpass filter dependent only on whether a registered level is above or below threshold so that a steady rate of gain adjustment, nonproportional to the amplitude, is obtained. Still another object is to improve the display system for the outputs of the bandpass filters.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a block diagram of a doppler detection apparatus according to the invention;

FIG. 2 is a schematic and block diagram of a level detector and gain compensator of FIG. 1; and

FIG. 3 is a schematic and block diagram of another embodiment of the level detector of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 a directional, single tone, continuous-wave acoustic signal generator 10, driven by a 30 kHz oscillator 11, provides the carrier signal which ensonifies the guarded volume of water (not shown). A directional acoustic signal receiver 12 receives reflected sonic waves originating from generator 10. The output of receiver 12 is fed via an audio amplifier 13 to a bandpass filter 14 having a center frequency preferably equal to that of the carrier signal and a bandwidth of about 200 Hz so that the doppler-shifted signal spectra may be processed. The output of filter 14 is passed to a doppler shift detector 16 which is'connected to oscillator 11 to utilize the original 30 kHz carrier frequency as a reference signal. Detector 16 in effect subtracts the carrier signal from the received signal producing an output which includes the complex doppler shift spectra introduced by the moving medium and reflective targets. The output of detector 16 is applied to a broad bandpass, variable gain control, audio amplifier 17 having an automatic gain control loop 18 to compensate for shifts in the average ambient level throughout all frequencies in the output of detector 16. The effect of automatic gain control 18 is to hold the overall level of the signal constant without regard to any particular frequency band. A conventional gain control having a rate of change proportional to the amplitude would be satisfactory for this purpose. 7 The output of amplifier 17 is passed via amplifier 19 toa modulator 21 which mixes the unfiltered composite doppler signal with a 300 Hz continuous-wave signal provided by an oscillator 22. The mixed output from modulator 21 is fed to a frequency meter 23 which is calibrated in velocity units such as knots. A

suitable display device for meter 23 is a vibrating reed frequency meter which displays amplitude and frequency information. The range of doppler frequencies involved is typically about 7 to Hz. For use with the vibrating reedtypemeter, the range from minimum to maximum frequency should not span more than one octave. Moreover, low frequency reeds are subject to mechanical vibration interference. Thus it is preferable to-mix the composite signal with a higher frequency secondary signal to shift upwardly the frequency range applied to the meter. 1f oscillator 22 supplies a 300 Hz signal, the sum frequency range would typically be 307 to 385 Hz. The information displayed on meter 23 represents both the river velocity, that is, the ambient level, and also swimming and floating targets which are within the guarded volume of water.

The output of amplifier 17 carrying the composite doppler shift frequencies is fed to a plurality of parallel narrow bandpass filters 24 having adjacent bandwidths. For example filter No. 1 may have a bandwidth of -10 Hz and filter No. 2 a bandwidth of -15 Hz, the amplitudes passed by each filter being down 3 db at the upper and lower ends of the respective bandwidths. The other filters, through, for example, No. 12, would have successive bandwidths to cover the normal range of doppler frequencies encountered. The output of each filter 24 is passed to a corresponding amplifier 26 having voltage-controlled variable gain. The output level of each amplifier 26 indicates the relative amplitude of acoustic signals within the band passed by the associated filter 24. For each band, a level detector 27 receives the output of amplifier 26. The twelve outputs of detectors 27, including those not shown, are fed to OR gate 29 which energizes an alarm 28 whenever the amplitude within any filtered band exceeds a predetermined level. Detector 27 also regulates a gain compensator 31 to adjust the gain of amplifier 26. Amplifier 26, detector 27, and compensator 31 may be viewed together as forming a control circuit for the corresponding filter 24.

Detector 27 and compensator 31 are shown in more detail in FIG. 2. The output of amplifier 26 is passed to' a rectifier 32 which supplies current to a coil 33 causing corresponding angular displacement of a pivoted pointer 34 in the meter 36. Meter 36 is furnished with

adjustable contacts

37 and 38 corresponding to threshold and alarm level amplitudes respectively of the output of amplifier 26. Compensator 31 includes an RC circuit having a variable resistor 41 with one portion connected to ground and another portion connected in series with a grounded capacitor 42. When the output amplitude of amplifier 26 is so low that pointer 34 touches contact 37, an electrical connection is established between a source of DC. voltage 39 and compensator 31 whereby resistor 41, capacitor 42 and DC. source 39 form a series circuit. The lead supplying voltage to control the gain of amplifier 26 is connected at the junction of resistor 41 and capacitor 42. The voltage output of compensator 31 increases as capacitor 42 is charged. The charge rate may be varied in accordance with the RC time constant in a manner well known to those skilled in the art. A suitable time constant would be about 5 minutes, commensurate with the normallyslow change in stream velocity. If contact between pointer 34 and contact 37 is broken by an increase in the output of amplifier 26, capacitor 42 will begin to discharge slowly through the grounded portion of resistor 41, thus reducing the voltage output from compensator 31 and decreasing the gain accordingly. The discharge rate of capacitor 42 is independent of the specific output amplitude of amplifier 26. Even for an alarm level signal within the associated band causing pointer 34 to touch contact 38, the gain decrease in amplifier 26 during discharge will be dependent only on the RC time constant of compensator 31. Contact 38 is connected to OR gate 29 (FIG. 1). When pointer 34 meets contact 38, voltage is supplied by DC. source 39 to activate alarm 28, indicating a predetermined alarm level within at least one filtered band.

Another arrangement suitable for carrying out the function of level detector 27 is shown in FIG. 3. The output from amplifier 26 is fed to

comparators

43 and 44 which may be biased transistors. Reference voltage for the

comparators

43 and 44 is supplied by series- 7 connected

variable resistors

46 and 47 which divide the voltage from DC. source 48. When the amplified signal falls below the reference voltage of resistor 46, comparator 43 provides a voltage output to compensator 31 through resistor 41 which increases the gain by charging capacitor 42. Comparator 44 likewise provides an output voltage to OR gate 29 when the output of amplifier 26 exceeds the alarm level established by resistor 47.

Referring to FIG. 1 the outputs of amplifiers 26 are passed via corresponding isolation amplifiers 51 to modulator 52 which receives the composite spectra associated with each narrow bandpass filter 24. Modulator 52 is similar to modulator 21 and mixes the doppler shift frequencies with the higher frequency, secondary signal supplied by oscillator 22. A frequency meter 53 is connected to modulator 52 and receives and displays the doppler frequencies passed by filters 24. Like the meter 23, the filtered frequency meter 53 is calibrated in velocity units. A second reed frequency meter may be suitably employed for meter 53. If two reed meters are used, it is desirable to mount one immediately above the other to facilitate comparison.

In operation generator 10 and receiver 12 are laterally spaced on a river bottom and aimed along the direction of the primary current to establish a guarded volume of water. A river current produces an ambient doppler spectrum at the output of shift detector 16. At any given time the relative amplitudes for the frequencies within this spectrum vary. For a one-knot current, the maximum amplitude will typically occur at 20 Hz. The shape of the amplitude vs. frequency spectrum will always be faithfully reproduced by meter 23 because its input is unfiltered. Under the same ambient conditions, the input to meter 53 will contain the same frequency spectrum, but the amplitudes for each frequency will be approximately equal; no maximum amplitude peak will be present at ambient levels.

Constant amplitude throughout the ambient spectrum is established and enforced by automatic compensation of the gain on each amplified filter when the contacts 37 within level detectors 27 are set for the same threshold. Assuming no target objects are present and the river velocity is constant, the pointer 34 will undergo small fluctuations frequently touching threshold contact 37. Due to compensator 31, the gain on amplifier 26 will be in a constant state of flux, either decreasing or increasing. When the output of the amplifier 26 is low enough for pointer 34 to touch contact 37, the gain will be slowly increased until pointer 34 breaks contact with contact 37. However, when pointer 34 is not completing the compensator circuit, the gain on amplifier 26 will be decreasingdue to the discharge of capacitor 42. Thus, pointer 34 will intermittently fall back to contact 37. With an increase in stream velocity, the ambient level represented by the output of amplifier 26 will rise forcing pointer 34 to an intermediate position within meter 36. At the same time compensator 31 will be decreasing the gain to bring pointer 34 back into intermittent touching of contact 37.

If the current decreased from 1 knot to A. knot, the corresponding peak doppler frequency on meter 23 would decrease from 20 to Hz. As a result, the output of the filter associated with Hz would decrease. If the gain on the associated filter amplifier 26 were not increased, doppler signals within the band from true targets might fail to be of sufficient amplitude to sound the alarm. If instead the current rose from 1 to 2 knots, the corresponding peak would shift to 40 Hz. The output of the filter handling 40 Hz would be increased to the extent that small increments in ambient level might trigger a false alarm. Therefore, the gain should be reduced to bring the ambient level of the affected filter back into agreement with the other filter outputs.

In conventional gain controls, the rate of change of the gain is proportional to the amplitude of the output signal. This type of gain control was found unsatisfactory for use with filter amplifiers 26 since rapidly reducing the gain might prevent a true target signal from reaching the alarm level. In the system of FIG. 1, however, when a moving reflecting object enters the ensonified volume, the'affected filter output rises much more rapidly than the associated gain compensator 31 can follow, thus allowing the amplitude to obtain the alarm level. By constantly correcting the ambient output level within each band to maintain a predetermined threshold, a fixed relationship between ambient conditions and the alarm level is assured thus, in effect, normalizing the sensitivity of each doppler filter over the full range of ambient conditions.

The number of narrow bandpass filters employed can be varied up to any practicable number depending on the selectivity desired. Compensator 31 and detector 27 are not limited to use in swimmer and floater detectors. They are inherently useful in any device which must detect the occurrence of transient amplitudes which bear a fixed relationship to a steadily changing ambient level.

The improved swimmer and floater detector frees sentries from the task of continuously monitoring the ambient levels within each filtered band. Manual ad justment during operation is unnecessary due to the normalizing effect of gain compensator 31 and detector 27. In addition, the use of a single filtered frequency meter for display permits adjacent location with the unfiltered display for simple comparison and enables direct reading of the actual doppler frequencies produced.

It will be understood that various changes in the details, materials, steps .and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the-invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

What is claimed is:

l. A control circuit comprising:

level detector means adapted to receive an amplifier output for providing a supply voltage at a first output terminal exclusively when the amplifier output is below a predetermined threshold and at a second output terminal exclusively when the amplifier output is a predetermined increment above the threshold, the second output terminal being adapted to be connected to an external alarm; and RC circuit means having an input terminal connected to the detector first output terminal, and an output terminal adapted to be connected to a feedback input terminal of the amplifier for providing a continuously increasing or decreasing voltage in the presence or absence respectively of the supply voltage at the level detector first output terminal.

2. The circuit of claim 1 wherein:

the level detector means comprises a meter having a pointer member whose displacement along a path corresponds to the level of the amplifier output, a voltage source connected to the pointer member, and first and second contact members located in the path at respective first and second positions corresponding respectively to the first and second predetermined levels, the first and second contact members being connected respectively to the first and second output terminals of the level detector, whereby voltage is supplied to the RC circuit or to the level detector second output terminal when the pointer member is in contact with the first or the second contact member respectively. 3. An automatic compensated level detection system for operating an external alarm, comprising:

amplifier means for receiving an electrical signal; a voltage source; switch means controlled by the output signal of said amplifier means for feeding said voltage source to a first output exclusively when'said amplifier output signal is below a predetermined amplitude, and to a second output exclusively when said amplifier output signal is a preselected increment above the predetermined amplitude for energizing the external alarm; and RC circuit means operatively connected between the switch means first output and a feedback input of said amplifier means for increasing the gain thereof when said voltage source is connected to said first output. 4. An automatic compensated level detection system according to claim 3, further comprising:

said amplifier means including an amplifier for receiving said electricalsignal and connected to said RC circuit, and a rectifier connected at the input to said amplifier for rectifying the output signal therefrom and connected at the output to said switch means. 5. An automatic compensated level detection syste according to claim 4, further comprising:

said switch means including an armature connected to said voltage source and movable between two output terminals, and a coil connected to the output of said amplifier for moving said armature in response to the amplifier output signal, said terminals being connected respectively to said switch means first and second outputs. 6. An automatic compensated level detector according to claim 3, further comprising:

said switch means including first and second comparator means operatively connected to said voltage source and said amplifier means output signal,

voltage source for providing a voltage of v preselected level, and a comparator receiving said voltage and the amplifier means output signal, the output of each of said comparators being connected to said switch means first and second outputs.

Claims

1. A control circuit comprising: level detector means adapted to receive an amplifier output for providing a supply voltage at a first output terminal exclusively when the amplifier output is below a predetermined threshold and at a second output terminal exclusively when the amplifier output is a predetermined increment above the threshold, the second output terminal being adapted to be connected to an external alarm; and RC circuit means having an input terminal connected to the detector first output terminal, and an output terminal adapted to be connected to a feedback input terminal of the amplifier for providing a continuously increasing or decreasing voltage in the presence or absence respectively of the supply voltage at the level detector first output terminal.

2. The circuit of claim 1 wherein: the level detector means comprises a meter having a pointer member whose displacement along a path corresponds to the level of the amplifier output, a voltage source connected to the pointer member, and first and second contact members located in the path at respective first and second positions corresponding respectively to the first and second predetermined levels, the first and second contact members being connected respectively to the first and second output terminals of the level detector, whereby voltage is supplied to the RC circuit or to the level detector second output terminal when the pointer member is in contact with the first or the second contact member respectively.

3. An automatic compensated level detection system for operating an external alarm, comprising: amplifier means for receiving an electrical signal; a voltage source; switch means controlled by the output signal of said amplifier means for feeding said voltage source to a first output exclusively when said amplifier output signal is below a predetermined amplitude, and to a second output exclusively when said amplifier output signal is a preselected increment above the predetermined amplitude for energizing the external alarm; and RC circuit means operatively connected between the switch means first output and a feedback input of said amplifier means for increasing the gain thereof when said voltage source is connected to said first output.

4. An automatic compensated level detection system according to claim 3, further comprisiNg: said amplifier means including an amplifier for receiving said electrical signal and connected to said RC circuit, and a rectifier connected at the input to said amplifier for rectifying the output signal therefrom and connected at the output to said switch means.

5. An automatic compensated level detection system according to claim 4, further comprising: said switch means including an armature connected to said voltage source and movable between two output terminals, and a coil connected to the output of said amplifier for moving said armature in response to the amplifier output signal, said terminals being connected respectively to said switch means first and second outputs.

6. An automatic compensated level detector according to claim 3, further comprising: said switch means including first and second comparator means operatively connected to said voltage source and said amplifier means output signal, the outputs of said first and second comparator means being connected respectively to said switch means first and second outputs.